This invention relates to a text processing system such as electronic typewriters and word precessors, particularly equipped with means for erasing characters which have been erroneously printed on a print sheet.
Some of the conventional text processing system are provided with character erasing means for erasing erroneously printed characters. The said character erasing means include a correction buffer for storing data of respective characters printed on the print sheet by operation of character keys. The correction buffer may also store shift data indicating that characters be printed on a shifted line parallel to the standard print line with a predetermined interval. Generally, such shift data is inputted when a superscript key or a subscript key is operated.
FIG. 5 illustrates a manner in which an equation of "XY.sup.2 -YX.sup.3 =Z" is printed with the conventional typewriter equipped with the correction buffer described above. More particularly, after characters "X" and "Y" are sequentially inputted by operation of corresponding character keys, a superscript key and a character key designating a figure "2" are sequentially operated to print the superscript "2" on the superscript print line L.sub.1. Then, a subscript key is operated to return the print head to the standard print line and correspondng keys are operated to print "-", "Y" and "X" on the standard print line L.sub.0. The superscript key and the figure "3" key are then sequentially operated to print a superscript "3". Finally, the subscript key is operated to return the printing head to the standard print line L.sub.0 on which "=" and "Z" are successively input.
As appeared from the above explanation, the superscript and subscript keys are used to drive a line feed mechanism or a platen which feeds a print sheet P in directions U and D, respectively. Thus, the characters or symbols "X", "Y", "-", "Y", "X", "=" and "Z" are printed on the standard print line L.sub.0, while the superscripts "2" and ""3" are printed on the shifted superscript print line L.sub.1 spaced by half a line apart from the standard print line L.sub.0. The respective character data and shift data are inputted to the correction buffer in response to each operation of the character key and the shift key including superscript and subscript keys.
Provided that "X" of "XY.sup.2 " has been erroneously printed and should be replaced by another character, which is found after the entire printing operation of this equation is completed. To carry out correction of the misprinted character "X", it is required to return the print head to the position of "X" by repeated operation of a backspace key, and after deletion of the mistypewritten character "X" and insertion of another desired character at the same position, the print head is moved to the print position next to the character "Z" by repeated operation of a space key, so that the typewriter will become ready for the succeeding printing operation.
These backward and forward movements of the print head required before and after the correction operation will be controlled by the character data and the shift data stored in the correction buffer, and the print head is moved just along the path B traveling during the initial printing operation. For example, when the backspace key is operated at the time the print head is positioned on "=", in response to the subscript shift data and character data both stored in the correction buffer for the subscript number "3", the print head is moved back along the standard print line L.sub.0, and immediately thereafter the print sheet P is fed in the direction D so that the print head is positioned just above the figure "3" printed on the superscript print line L.sub.1. When the backspace key is operated once more, likewise, the print head is moved to a position juxtaposed on "X" responsive to the superscript shift data and the character data stored in the correction buffer by moving the print head one character back and by feeding the print sheet P in the direction U. Also, during the forward movement of the printing head after erasion of "X" and printing of another character, the line feed mechanism is caused to drive responsive to the subscript shift data stored in the correction buffer for the figure "3" and the print head is moved forward just along the path B traveling during the initial printing operation.
Consequently, the correction operation carried out with the conventional typewriters inevitably includes the steps of feeding the print sheet when the superscript or subscript has been printed between the current carriage position and the position of the character to be erased. Such paper feeding step will not be inherently required in order to erase the character printed on the same print line on which the print head is now located. As a result, the print sheet P has been very likely to be displaced from the correct print line during such backward and forward movements of the print head, under the influence of backlash of gears of the line feed mechanism. To prevent the influence of backlash of the gears, an additional control program for controlling the sheet feed should be provided. A complicated control program and an increased storage capacity for the correction program have been required for driving the line feed mechanism in accordance with the shift data stored in the correction buffer.